Depends what I am drawing, the following two I often listen to when doing coastlines or bodies of water.

http://www.youtube.com/watch?v=YSMx5YQ0a-U

http://www.youtube.com/watch?v=UQeqmNbA2Hs

Mountains:

http://www.youtube.com/watch?v=sfR_HWMzgyc

http://www.youtube.com/watch?v=FWiyKgeGWx0